Granitic rocks

Granitic rocks are coarse-grained igneous rocks consisting mainly of quartz, sodic plagioclase, and alkali feldspar, with various accessory minerals. These rock types occur primarily as large intrusive bodies that have solidified from magma at great depths. Granitic rocks can also occur to a lesser degree as a result of metamorphism, a process referred to as granitization.

Granitic Rock Occurrence

The term “granitic rocks” generally refers to the whole range of plutonic rocks that contain at least 10 percent quartz. They are the main component of continental shields and also occur as great compound batholiths in folded geosynclinal belts. Granitic rocks are so widespread, and their occurrence and relation to the tectonic environment are so varied, that generalizations often obscure their complexity. Major granitic complexes are, in general, a continental phenomenon occurring in the form of batholiths and migmatite complexes.

When large masses of magma solidify deep below the ground surface, they form igneous rocks that exhibit a coarse-grained texture described as phaneritic. These rocks have the appearance of being composed of a mass of intergrown crystals large enough to be identified with the unaided eye. A large mass of magma situated at depth may require tens of thousands or even millions of years to solidify. Because phaneritic rocks form deep within the crust, their exposure at ground surface reflects regional uplift and erosion, which has removed the overlying rocks that once surrounded the now-solidified magma chamber.

Along continental margins, belts of granitic rocks developed as batholiths composed of hundreds of individual plutons. The formation of batholithic volumes of granitic magma generally appears to require continental settings. Some of the more prominent batholiths in North America are the Coast Range, Boulder-Idaho, Sierra Nevada, and Baja California. The largest are more than 1,500 kilometers long and 200 kilometers wide, and have a composite structure. The Sierra Nevada, for example, is composed of about 200 plutons separated by many smaller plutons, some only a few kilometers wide.

Granitic Rock Classification

As with other rock types, granitic rocks are classified on the basis of both mineral composition and fabric or texture. The mineral makeup of an igneous rock is ultimately determined by the chemical composition of the magma from which it crystallized. Feldspar-bearing phaneritic rocks containing conspicuous quartz (greater than 10 percent in total volume) in addition to large amounts of feldspar can be designated as granitic rocks. This nonspecific term is useful where the type of feldspar is not recognizable because of alteration or weathering, for purposes of quick reconnaissance field studies, or for general discussion.

Granitic rocks consist of two general groups of minerals: essential minerals and accessory minerals. Essential minerals are those required for the rock to be assigned a specific name on a classification scheme. Essential minerals in most granitic rocks are quartz, sodic plagioclase with 30 to 50 percent calcic plagioclase, and potassium-rich alkali feldspars (either orthoclase or microcline). Accessory minerals include biotite, muscovite, hornblende, and pyroxene.

When an initial phase of slow cooling and crystallization at great depths is followed by more rapid cooling at shallower depths or at the surface, porphyritic texture develops, as is evident in the presence of large crystals enveloped in a finer-grained matrix or groundmass. The presence of porphyritic texture is evidence that crystallization occurs over a range of temperatures, and magmas are commonly emplaced or erupted as mixtures of liquid and early-formed crystals.

Classification of granitic rocks can be based either on the bulk chemical composition or on the mineral composition. Because oxygen is ubiquitous in rocks, chemical compositions are given as the weight percentages of oxides rather than individual elements, whereas mineral composition units are in approximate percentages in total volume. The mineral composition of granitic rocks provides a reliable basis for classification. Specific granitic rock types are defined on the basis of the presence of quartz and the ratio of potassium feldspar and plagioclase. The presence of other accessory minerals may be indicated in the form of a modifier (such as biotite granite).

Granitic Rock Groups

Granitic rocks include true granites, tonalite, granodiorite, quartz monzonite granite, soda granite, and vein rocks of pegmatite and aplite. The mineralogy of these varies, and the distinction between different granitic rocks can be gradational, but all are defined by the presence of quartz. Tonalite has mostly plagioclase feldspar and very little potassium feldspar. Granodiorite is composed predominantly of plagioclase feldspar, with subordinate potassium feldspar and biotite, hornblende, or both as accessory minerals. Quartz monzonite is composed of approximately equal amounts of potassium and plagioclase feldspars, with biotite, hornblende, or both as accessory minerals. Granite is composed predominantly of potassium feldspar with subordinate plagioclase feldspar and biotite alone or with hornblende or muscovite as accessory minerals. Soda granite is composed predominantly of very sodium-rich plagioclase feldspar, with small amounts of sodium-bearing pyroxene or amphibole.

Pegmatite is a very coarse-grained complex rock in terms of mineralogy. Structurally, pegmatites occur as dikes associated with large plutonic rock masses. Dikes are tabular-shaped intrusive features that cut through the surrounding rock. The large crystals are inferred to reflect crystallization in a water-rich environment. Most pegmatites are simple in terms of mineralogy, consisting primarily of quartz and alkali feldspar, with lesser amounts of muscovite, tourmaline, and garnet; they are referred to as simple pegmatites. Other pegmatites can be very complex and contain other elements that slowly crystallize from residual, deeply seated magma bodies. High concentrations of these residual elements can result in the formation of minerals such as topaz, beryl, and minerals rich in rare-earth elements, as well as quartz and feldspar. Aplite is a very fine-grained, light-colored granitic rock that also occurs as a dike and consists of quartz, albite, potassium feldspar, and muscovite, with almandine garnet as an occasional accessory mineral.

Other rocks that are occasionally grouped with granitic rocks are migmatites. Migmatites, or mixed rocks, are heterogeneous granitic rocks that, on a large scale, occur within regions of high-grade metamorphism or as broad migmatite-rich zones bordering major plutons. Migmatites appear as alternating light and dark bands. The light-colored bands are broadly granitic in mineralogy and chemistry, while the darker bands are clearly metamorphic. Migmatites are believed to reflect the partial melting of metamorphic rocks in which the portions rich in quartz and feldspar melt first.

Granitic rocks are classified by the relative proportions of quartz, plagioclase, and potassium feldspar, and can also be classified by their aluminum content. Peraluminous granites have excess aluminum present after the formation of feldspar, and thus contain aluminum-rich minerals like sillimanite or garnet. Peralkaline granites run out of aluminum before using up their sodium and potassium and have alkali-rich minerals like sodium amphibole. Granitic rocks can further be divided into two groups, S-types and I-types, according to whether they were derived from predominantly sedimentary or igneous sources, respectively. Enormous volumes of I-type granites, which constitute most plutons, occur along continental margins overriding subducting oceanic material.

Inclusions

A common feature of granitic rocks, notably in granodiorite to dioritic plutons, is the presence of inclusions or rock fragments that differ in fabric and/or composition from the main pluton itself. The term “inclusions” indicates that they originated in different ways. Foreign rock inclusions, called xenoliths, include blocks of wall rocks that have been mechanically incorporated into the magma body. This process is referred to as stoping. Some mafic inclusions are early-formed crystals precipitated from the magma itself after segregation along the margin of the pluton, which cools first. These inclusions are called antoliths. Other inclusions may reflect clots of solidified mantle-derived magma that ascended into the granitic source region, or residual material that accompanied the magma during its ascent.

Petrographic Study

Granitic magmas can be derived from a number of sources, notably the melting of continental crust, the melting of subducted oceanic crust or mantle, and differentiation. The problem facing the geologist is to decide which tectonic environment led to the formation of the granite. To accomplish this objective, both petrographic and geochemical information is used.

The standard method of mineral identification and study of textural features and crystal relationships is the use of rock thin sections. A thin section is an oriented wafer-thin portion of rock 0.03 millimeter in thickness that is mounted on a glass slide. The rock thin section is so thin that even normally opaque minerals transmit light and reveal mineral content, abundance and association, grain size, structure, and texture. The thin section also provides a permanent record of a given rock that may be filed for future reference.

Thin sections are studied with the use of a petrographic microscope, which is a modification of the conventional compound microscope commonly used in laboratories. The modifications that render the petrographic microscope suitable to study the optical behavior of transparent crystalline substances are a rotating stage, an upper polarizer or lower polarizer, and a Bertrand lens, which together are used to survey the optical properties of the mineral over a wide range of angles at once. With a magnification that ranges from about 30 to 500 times, the petrographic microscope allows one to examine the optical behavior of transparent crystalline substances or, in this case, crystals that make up granitic rocks.

The study of granitic rocks may be greatly facilitated by various staining techniques of both hand specimens and thin sections. Staining is employed occasionally to distinguish potassium feldspar from plagioclase and quartz, the three main mineral constituents of granitic rocks. A flat surface on the rock is produced by sawing and then polishing. The rock surface is etched using hydrofluoric acid, an extremely hazardous material that requires great care in handling. This step is followed by a water rinse and immersion in a solution of sodium cobaltinitrate. The potassium feldspars will then turn bright yellow. After rinsing with water and covering the surface with rhodizonate reagent, the plagioclase becomes brick red in color. Staining techniques are available for other minerals, including certain accessory minerals such as cordierite, anorthoclase, and feldspathoids.

Measurement of the relative amounts of various mineral components of a rock is called modal analysis. The relative area occupied by the individual minerals is estimated or measured on a flat surface (on a flat-sawed surface, on a flat outcrop surface, or in thin section) and then related to the relative volume. Caution must be used, because the relative area occupied by any mineral species on a particular planar surface is not always equal to the modal (volume) percentage of that mineral on the rock mass, especially if mineral grains are clumped or strongly aligned.

Geochemical Study

When a rock specimen is crushed to a homogeneous powder and chemically analyzed, the bulk chemical composition of the rock is derived. Chemical analyses are normally expressed as oxides of the respective elements because of the overwhelming abundance of oxygen. Analysis of granitic rocks shows them to be typically rich in silica potassium and sodium, with lesser amounts of basic oxides such as magnesium, iron, and calcium oxides. Magnesium, iron, and calcium oxides are present in higher abundance in basalts, which contain plagioclase feldspar, pyroxene, and olivine.

Isotopes such as those of strontium, oxygen, and lead can also be used as tools in evaluating granitic magma sources, such as a mantle origin and crustal melting of certain rock types. Some accessory minerals reflect the trace element content of the magma and thus the possible nature of their source. Much research is focused on chemical tracers. Tracers help distinguish the source region of a granitic magma, such as lower-crustal igneous or sedimentary rock or mantle material.

Industrial Applications

Granitic rocks have been used as dimension stones for many years. Dimension stones are blocks of rock with roughly even surfaces of specified shape and size used for the foundation and facing of expensive buildings. When crushed, granitic rocks can be used as aggregate in the cement and lime industry. In addition to these uses, granitic rocks are valued because of their geographic association with gold. Gold ores are found in close proximity to the contacts of the granitic bodies within both granitic rocks and surrounding rocks.

Pegmatites can also be very valuable. Simple pegmatites are exploited for large volumes of quartz and feldspar, used in the glass and ceramic industries. Complex pegmatites can also be a source of gem minerals, including tourmaline, beryl, topaz, and chrysoberyl. In spite of varied mineral composition, relatively small size, and unpredictable occurrence, pegmatites constitute the world’s main source of high-grade feldspar, electrical-grade mica (used for supporting heater elements in toasters), certain metals (including beryllium, lithium, niobium, and tantalum), and some piezoelectric quartz, although most industrial quartz is now grown synthetically rather than mined.

Principal Terms

aphanitic: a textural term that applies to an igneous rock composed of crystals that are microscopic in size

crystal: a solid made up of a regular periodic arrangement of atoms

crystallization: the formation and growth of a crystalline solid from a liquid or gas

granitization: the process of converting rock into granite; it is thought to occur when hot, ion-rich fluids migrate through a rock and chemically alter its composition

isotopes: atoms of the same element with identical numbers of protons but different numbers of neutrons, thus giving them a different mass

magma: a body of molten rock typically found at great depths, including any dissolved gases and crystals

migmatite: a rock exhibiting both igneous and metamorphic characteristics, which forms when light-colored silicate minerals melt and crystallize, while the dark silicate minerals remain solid

phaneritic: a textural term that applies to an igneous rock composed of crystals that are macroscopic in size, ranging from about 1 to over 5 millimeters in diameter

pluton: a structure that results from the emplacement and crystallization of magma beneath the surface of the earth

porphyritic: a texture characteristic of an igneous rock in which macroscopic crystals are embedded in a fine phaneritic or aphanitic matrix

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